This invention relates generally to RF power amplifiers for wireless communications, and more particularly, the invention relates to microwave and millimeter wave integrated circuit (MMIC) power amplifiers having output impedance matching networks.
The trend of wireless communication equipment has led to an increasing demand for high performance and small form factor electronic components. In the modern wireless application, the radio frequency (RF) power amplifier circuit is one of the largest components in the transceiver portion of the phone. Thus the reduction of its size has been targeted in the industry.
The microwave and millimeter wave integrated circuit (MMIC) power amplifier is typically impedance matched to 50 ohms at both the input and output. While the amplifier is usually operated at 10 GHz and above, the output matching circuit at lower frequencies becomes lossy. The output network of a MMIC circuit uses metal-insulator-metal (MIM) capacitors and metal traces on the surface of the semiconductor substrate as the transmission line. The losses in the transmission line increase per unit electrical length as frequency is lowered. Therefore the loss of the output matching circuit for RF and low microwave frequency is intolerable. Thus while the MMIC power amplifier offers small size, the losses in the impedance matching network make it impractical at low microwave frequencies.
To improve RF performance while maintaining a small form factor, a power amplifier module (PAM) is used. FIG. 1 is a block diagram of a conventional power amplifier module for RF/low microwave frequency applications. The module includes a power amplifier integrated circuit (PAIC) 10 which is connected to a separate output matching circuit 12. As shown in FIG. 2, the output matching circuit typically comprises a planar transmission line (TL) or microstrip 14 and surface-mount-technology (SMT) capacitors 16 which are mounted on a ceramic or laminate substrate 18 along with the integrated circuit 10.
The PAIC integrated circuit is usually the full MMIC power amplifier less the lossy output matching circuit. To reduce the transmission line loss in the PAM, the line width of the TL must be increased which in turn requires longer physical length to maintain the original inductive effect. Therefore, the module size and TL loss become trade-offs in the PAM design. Compared with the transmission line on the MMIC power amplifier, the transmission line on a power amplifier module has much thicker metal (3 to 5 microns on the MMIC compared to 25-50 microns on the PAM substrate) and therefore offers much less loss. Another way to improve conversion loss is the use of high quality factor (Q) SMT capacitors. However, a commercial high quality capacitor is not cost effective and its Q is always lower than that available in the integrated circuit.
In accordance with the invention, an RF power amplifier includes an integrated circuit in which an amplifier circuit is fabricated and an output impedance matching circuit including at least one capacitor mounted on the integrated circuit substrate with bonding wire inductors connecting the amplifier circuit to the at least one capacitor. In a preferred embodiment, the capacitor is mounted on a major surface of the integrated circuit semiconductor body in a hybrid integrated circuit.
Preferably the one or more capacitors of the output impedance matching circuit comprise metal-insulator-metal (MIM) capacitors with wire bonding serially connecting the integrated circuit output and the capacitors. The length and height of the wire bonds can be selected to provide the requisite impedance for a particular operating frequency of the power amplifier.
The form factor of the power amplifier is reduced since the bond wire inductor occupies less chip surface area than the conventional transmission line inductor. Further, the wire bond inductor has lower loss than the planar transmission line, and multiple bonding wires can be used to further reduce the loss. The resulting hybrid power amplifier has the combined strength of the MMIC power amplifier and the PAM module in performance yet offers low-cost manufacturing through use of conventional semiconductor fabrication techniques.
The invention and object features thereof will be more readily apparent from he following detailed description and dependent claims when taken with the drawing.